This invention relates to activated carbon fiber with anion-exchange and cation-exchange groups on its surface, and a process for preparing the same. More specifically, it relates to activated carbon fiber obtained by activating carbonized cellulose fiber which has been pretreated with a solution containing urea and/or organic salts of ammonia at an elevated temperature in an atmosphere containing steam and/or carbon dioxide. The thus obtained activated carbon fibers have anion-exchange and complex forming groups on its surface and are especially suitable for removing organic and inorganic impurities from fluids.
Activated carbon fibers (referred to hereafter as ACF) are manufactured by activating carbonized fiber at an elevated temperature in an activating gas atmosphere, typically steam and/or carbon dioxide and/or ammonia. Carbonized fibers are made by carbonizing polyacrylonitrile, phenol resin, pitch or cellulose fibers in an inert atmosphere. In conventional carbonization, the organic material is heated to 200.degree. to 800.degree. C., typically over 400.degree. C. for sufficient time to remove low molecular weight organics and tars leaving more than 90% carbon, typically in the form of crystalline-amorphous structures (graphite layers) rather than a porous structure. Use of steam or CO.sub.2 is avoided to retain the carbon fiber strength. Pretreatment steps prior to carbonization are known in the art.
Processes for producing fibrous activated carbon have been known for many years wherein fibrous organic materials are first carbonized in an inert atmosphere to remove volatile materials, and then activated to form the desired porous active surface in the carbonized fiber. Activated carbon fibers are very small in diameter, typically 5 to 30 microns. Very small fiber diameter provides high adsorption capacity and rate of adsorption. At the same time, very small fiber diameter makes it more difficult for carbon fiber to retain its integrity after activation (over-activated carbon fiber may easily turn into powder). In order to improve the flexibility of carbonized fibers, U.S. Pat. No. 4,409,125, teaches performing the carbonization process in the presence of ammonium chloride, nitric acid or boric acid. The carbonized fiber is then activated with zinc chloride.
ACF main advantages over powdered activated carbon are higher adsorption capacity, higher speed of adsorption and lesser compaction under flow. One of the disadvantages of powdered activated carbon is that it compacts during filtration (compaction under flow) leading to a sharply increased flow resistance. Powdered activated carbon can be bound into a porous rigid matrix, which reduces flow resistance, but it also limits the adsorption capacity.
Activated carbon fiber may be used for removing impurities either from gas or water.
A preferred adsorbent for tap water purification should have high adsorption capacity and high adsorption rate toward organic impurities, anions and cations. It should also be strong and flexible, so that it does not break down into the powder or compact significantly under water flow. It should also be inexpensive to produce. The present invention meets these needs.